1. Field of the Invention
The present invention relates to a means for measuring cloud altitude.
2. Prior Art
A device which is particularly suitable for measuring cloud altitude is known from U.S. Pat. No. 3,741,655. The known device comprises a measuring equipment of optical radar type, which means that an emitter unit emits short light pulses directed towards an object. In this case the object is a cloud and when such light pulses hit the cloud reflexes occur, and part of the reflected light is intercepted by a receiver unit located adjacent to the emitter. The time required for the light to travel the distance emitter-cloud-receiver is measured and the altitude of the cloud is determined with the known velocity of light as the starting point. The device known from the U.S. Pat. No 3,741,655 further comprises two integrating devices in the receiver unit, which are alternately caused to receive signals intercepted by the receiver unit. One of the integrating devices is designed to receive echo signals expected to be emitted from clouds, and the other integrating device is designed to receive noise signals only. After a number of light pulses have been emitted and echo signals have been received, the contents of the integrating devices are compared, and the result of the comparison is placed in proportion to a pre-determined signal level, and if this level is exceeded the existence of clouds is indicated.
In the event of rainfall from the clouds to which the altitude is to be measured, this will make the measurement considerably more difficult since echo signals are received also from the rain, that is the noise level increases and the cloud echo signals are then more difficult to distinguish. This problem will be described in more detail with reference to FIG. 1 which shows an example of echoes received by the receiver unit in the case of atmospheric precipitation.
The letter A designates an echo characteristic of atmospheric precipitation (fall of rain) with a decay time of t.sub.A. B designates a characteristic echo for a cloud with a decay time t.sub.B.
The receiver examines the existence of clouds stepwise at intervals with an extension of, for example, 5 meters in the range of 0-1000 meters. In this way one or more cloud veils lying over one another may be continuously registered up to 1000 meters, or to the height where the reflected light does no longer reach back to the receiver.
This measuring in steps is achieved because the receiving intervals for the two integrating devices mentioned are moved in parallel in time so that the whole measuring range is scanned.
According to the U.S. Pat. No. 3,741,655, the pulses controlling the opening of the integrating device, which receives only noise dependent on sun light and the noise generated by the amplifier itself, lie in time before the pulses which control the opening of the second integrating device, which is designed to receive echo signals from clouds. This means that the second integrating device all the time receives signals from a higher height above the ground than the signals received by the first integrating device. The cloud echo B naturally emanates from a higher height than the rain echo A in FIG. 1.
This means that, during the scanning of the measuring range, when echo signals from the cloud are received in the second integrating device, an echo signal derived from the rain under the cloud is received in the first integrating device, and the latter echo signal will completely or partly counterbalance the signal from the second integrating device when the contents of the two integrating devices are compared. This leads to difficulties in obtaining a sufficiently strong signal from the indicating device for certain types of weather.